Technically, demons aren't people. They only came from the emotions, separated from their spirit. They are left to simply exist and follow their harmful nature. The more powerful the emotion is, the easier it is for them to possess stronger objects. That is why the stronger the object is, the more powerful the demons are.
For example, the demons possessing mercury were very strong, but demons possessing water or ice are usually weaker. That is why they are categorized to be stronger, because they are capable of possessing stronger objects.
When Uncle Deve and Aunt Linda were still active professional exorcists, they met many kinds of demons. The most absurd one they found was demons possessing poo, and I wonder if I will ever experience going against a Poo demon.
However, the most powerful one they faced was the Ice demon. The one me and Nico destroyed last week with Aunt Linda watching.